1. Field of the Invention
The present invention relates generally to a PWM signal power amplifier and more directly relates to a PWM signal power amplifier which has a low distortion ratio in spite of fluctuations in the operating voltage.
2. Description of the Prior Art
In the art, it is typical to separate a pre-amplifier from a power amplifier. Recently, one arrangement contemplates that a power amplifier be disposed in the same cabinet as a loudspeaker and to electrically connect the output of a pre-amplifier to the input of the power amplifier in the loudspeaker by a lead wire. As disclosed in the U.S. Pat. No. 4,021,745 entitled "Pulse Width Modulated Signal Amplifier" which is assigned to the same assignee as that of this invention, a PWM signal which is in proportion to the instantaneous amplitude of an audio signal can be obtained with low distortion.
In the above, a PWM modulator is provided together with the pre-amplifier and that the output therefrom is transmitted through, for example, an optical fiber or the like, which exhibits less transmission loss, to the input of the power amplifier provided in the loudspeaker. In this case, it is desired that as a power amplifier for the PWM signal such an amplifier be used which is not deteriorated in its distortion characteristic particularly in the presence of fluctuation of a power supply voltage. However, up to now there has not been provided a satisfactory power amplifier for such a PWM signal.